Generally, a muddy substance having a high viscosity and containing a large amount of bubbles is difficult to pump by a centrifugal pump. Simple, safe means for pumping and transporting such a muddy substance further containing solid foreign matters has generally been desired. A conventional centrifugal pump cannot be simply used for the foregoing purpose even if the centrifugal pump is used in combination with a vacuum device because cavities formed in a region near the central part of the impeller by centrifugal separation cannot be easily replaced with a liquid having the foregoing properties.
Such a problem was solved clearly by an invention disclosed in JP-B-40-3655 relating to a centrifugal pump. As is apparent from JP-B-40-3655, this prior art centrifugal pump comprises, in combination, a main pump unit for pumping a liquid, and an auxiliary pump unit for removing cavities, having a suction opening which opens in the vicinity of a central part of the main impeller of the main pump unit and is disposed in parallel to the main pump unit, in which the suction opening of the auxiliary pump unit is formed in a small sectional area relative to the discharge ability of the auxiliary pump unit. The discharge opening of the auxiliary pump unit is opened into the suction side of the main pump unit. An exhaust passage is formed between a position near a central part of the auxiliary impeller of the auxiliary pump unit to a vacuum pump to remove forcibly cavities formed in the vicinity of the central part of the main impeller of the main pump unit so that the liquid can always be continuously pumped. A self-priming centrifugal pump disclosed in JP-B-42-3145 is an improvement of the invention disclosed in JP-B-40-3655. As shown in FIGS. 15 and 16, this prior art self-priming centrifugal pump is provided in its exhaust passage with a safety valve 6 to be operated by an actuator which is displaced by a negative pressure to prevent a vacuum (exhaust) pump 12 from failure in operation due to the flow of a liquid into the exhaust passage while the self-priming centrifugal pump is stopped. (Hereinafter, those invention will be referred to as "original inventions".)
This prior art device of the original inventions is capable of pumping up muddy substances which have been difficult to pump and has prevalently been used. However, the prior art device still has the following unsolved problems.
First, the prior art self-priming centrifugal pump relies on negative pressure produced by the vacuum pump to provide power to open the safety valve. Therefore, the negative pressure decreases when the safety valve is opened and the safety valve starts closing, and the negative pressure increases when the safety valve is closed and the safety valve starts opening. Thus, the safety valve repeats closing and opening operations to generate vibration and noise. It is possible that such a flapping action of the safety valve makes the operation of the safety valve unstable.
Secondly, any problem cannot arise because the discharge ability of the gas-liquid separating auxiliary pump unit surpassing the negative pressure produced by the vacuum pump isolates an exhaust system from the liquid while the self-priming centrifugal pump is in operation, and the safety valve is closed to isolate the exhaust system from the liquid while the pump is stopped. However, the discharge ability of the auxiliary pump unit becomes insufficient and the negative pressure produced by the vacuum pump overcomes the discharge ability of the auxiliary pump unit at a transient moment when the auxiliary pump unit operates at an operating speed lower than its normal operating speed, such as the moment the pump is started or the moment the pump starts stopping. Therefore, it is possible that the main pump unit communicates with the exhaust system, the liquid is sucked from the main pump unit into the vacuum pump by suction to contaminate the vacuum pump or to cause a failure in operation of the vacuum pump because the safety valve is half open at the transient moment. At the moment the pump starts stopping, it is possible that the liquid flows reverse into the vacuum pump for a moment if a high back pressure is acting on the discharge side of the main pump unit, or the working fluid of the vacuum pump (if the vacuum pump is of a liquid-seal type) is sucked into the main pump unit by a negative pressure produced by the reverse flow of the liquid if the suction lift of the main pump unit is high to cause working fluid loss and to contaminate the liquid.
Practically, problems arising from the instantaneous communication of the liquid with the exhaust system may be ignored when the self-priming centrifugal pump is used for pumping a common liquid, such as water, and the working fluid of the vacuum pump is water. However, if the self-priming centrifugal pump is used for pumping a liquid which must carefully be handled, such as a chemical liquid or a liquid foodstuff, both the contamination of the vacuum pump by the liquid and the contamination of the liquid by the working fluid of the vacuum pump are significant problems.
To solve such problems, the exhaust ability of the vacuum pump is reduced intentionally while the auxiliary pump unit is operating at an operating speed lower than its normal operating speed, or the self-priming centrifugal pump is provided with valves and cocks in addition to the safety valve and the additional valves and cocks are manually operated. However, these measures need complicated devices and troublesome work and the operation of the self-priming centrifugal pump falls far short of automatic operation. Thus, those measures could not have been substantial measures capable of solving those problems.
Accordingly, it is an object of the present invention to provide a durable self-priming centrifugal pump comprising a main pump unit and a vacuum device, having a simple construction, capable of drastically solving the foregoing problems in the prior art, employing new mechanisms and the like capable of stably and reliably functioning and maintaining the high performance of the self-priming centrifugal pump, capable of preventing the flow of liquids between the main pump unit and the vacuum device and of perfectly automatic operation, not requiring much work for maintenance, capable of being constructed in either a large size or a small size, and very economical in costs of equipment and maintenance.